1. Field of the Invention
The present invention relates to the control of high intensity discharge lamps, and particularly to the ignitor pulse starting circuit, required for the initial start up and operation of high intensity discharge (HID) lighting systems.
2. Discussion of the Art
Conventionally, several steps are involved in the start up and sustained operation of an HID lamp. A first step is inherent to lamp design and involves the reduction of arc tube internal gas pressure with respect to atmosphere. A second step, includes the application of a preselected voltage to a set of electrodes to initiate and sustain an arc discharge in the constituent gas of an HID arc tube. A third step, involves using an appropriate gas mixture, which is typically a combination of at least two gases, one of which makes up only about 1% of the total volume and is called the minor constituent. The minor constituent aids in the arc discharge and subsequent thermionic emission of the primary gas mixture, called the major constituent. In the process of operating an HID lamp, commonly, an ignitor circuit is used to generate high voltage pulses to ionize the gases and initiate the arc discharge. As an alternative, a starter electrode may be used to apply either a heating effect, or a high voltage, which aids in the generation of the arc discharge during start up. It is to be noted that a starter electrode is not commonly used with ignitors.
A problem encountered with the use of ignitor circuits, is an inherent wide variation of ignitor pulse heights. The variation exists due to differing component tolerances. Another obstacle is a lack of interchangeability between ballast types. There will, for example, be different inductive and capacitive loading of the ignitor pulse by ballasts from different manufacturers, as well as different pulse specifications and different ballast designs.
An ignitor pulse variable reduction method and apparatus, of the present invention overcomes the limitations of the prior art starting mechanisms by providing a universal ignitor for the different types of lamps and allows interchangeability of lamps, ignitors and ballasts between different lighting systems and manufacturers. While it is possible to match some components of a system to obtain better operational performance, the drawbacks include tighter manufacturing tolerances across all of these various components, and the associated increase in manufacturing costs.
The ignitor pulse variable reduction invention substantially reduces the requirement for tighter component tolerances by adding a voltage pulse clamping device which will ensure that the ignitor output voltage does not exceed the maximum lamp voltage rating, regardless of component tolerances or variations in supply line voltage. If the population of ignitor voltages is then set higher than the clamping voltage, all of the ignitors would then have approximately the same voltage level, due to the clamping device. The population of ignitor voltage levels, therefore, becomes a function of tolerance variations within the population of the clamping devices.